It is desirable for tire manufacturers to produce tires that have good wet skid resistance, low rolling resistance, tear strength, and good wear characteristics. It has traditionally been difficult to improve the wear characteristics of a tire without sacrificing wet skid resistance and traction characteristics. These properties depend, to a great extent, on the dynamic viscoelastic properties of the rubbers utilized in making the tire. Such viscoelastic properties are controlled largely by both the microstructure and macrostructure of the elastomer.
In order to reduce the rolling resistance and to improve the treadwear characteristics of tires, rubbers having a high rebound have traditionally been utilized in making tire tread rubber compounds. On the other hand, in order to increase the wet skid resistance of a tire, rubbers that undergo a large energy loss have generally been utilized in the tread of the tire. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber are commonly used as a rubbery material for automobile tire treads.
It has been conventionally believed to be desirable, e.g., for styrene-butadiene rubber, when utilized in tire tread compounds, to have a high level of vinyl content (1,2-microstructure) or a high level of styrene content, i.e., between 35% to 50% styrene content. In the case of high styrene content, it is desirable to have low vinyl content. To this end, styrene-butadiene rubbers are often synthesized by solution polymerization that is conducted in the presence of a catalyst system, including one or more modifying agents. In contrast to providing high vinyl or high styrene content rubbery polymers, it is believed that rubbery polymers having high trans microstructure contents may provide a more desirable balance of tire tread properties due to its crystallinity and low vinyl content. Polymerization of these rubbery polymers can be carried out utilizing batch, semi-continuous, or continuous techniques.
Batch processes are of considerable importance to the tire industry. A wide variety of certain types of polymers are manufactured in batch operations. Batch processes are typically used when production volumes are low, isolation is required for reasons of sterility or safety, and/or when the materials involved are difficult to handle. However, with recent trends in building small, flexible plants that are close to the markets of consumption, there has been renewed interest in batch processing. In the face of increased competition, process optimization is a natural choice for reducing production costs, improving product quality, reducing product variability, and ease of scale-up.
Thus, it would be desirable to provide an improved batch process for synthesizing rubbery polymers, such as styrene-butadiene rubber, having a high trans microstructure content, wherein the rubbery polymers can provide desirable wear properties without substantially sacrificing other performance characteristic(s), e.g., traction properties.